This invention relates to rotary indicating instruments of the type including, besides a regular indicating hand, a maximum indicating drag hand and associated manually operable resetting means.
Examples of such instruments are thermometers, pressure gages, and tachometers. In instruments of this type the maximum hand is pushed ahead around its axis by the regular hand as the latter is driven relative to a calibrated dial; and it retains its advanced position after the regular hand has fallen back towards its starting position. After a reading is taken of the advanced position of the maximum hand, the latter is resettable by the user to its starting or reference point.
Such instruments are used indoors and outdoors and accordingly are subjected to all kinds of weather. It is particularly desirable to avoid entry of moisture into the instrument because of its undesirable damaging effects upon the working parts and accuracy of the instrument.
The viewing crystal mounted over the front end of the casing of such instruments is customarily sealed against entry of moisture around its periphery. But the provision, as in known instruments, of an axial hole in the crystal for reception of the supporting shaft of the maximum hand to enable the latter to be reset by manipulative means located externally of the instrument is undesirable. A hole of this nature tends to allow seepage of undesirable moisture around the shaft to the interior of the instrument. Accordingly, to protect against such leakage in such mountings the expense and labor of providing a seal about the shaft and hole is needed. Further, in the process of drilling such a hole or assembling the shaft components in it renders the crystal subject to cracking or other damage.
A general objective of this invention is to avoid the foregoing faults so as to provide a moisture proof crystal.
A still further object of the invention is to provide for such instruments improved maximum hand reset means which insures against deviation of the maximum hand from its advanced position under undesirable vibrations to which the instrument may be subjected.
A further objective of this invention is to provide for instruments of this general nature an improved crystal element and an arrangement of the maximum hand relative to the crystal element together with externally operable magnetic means for resetting the maximum hand, whereby an axial hole through the crystal for the supporting shaft of the maximum hand is made unnecessary.
A feature of the present invention is an improved crystal for the instrument, one having a solid transparent body which is provided with integral stub shafts extending axially from opposite faces thereof, upon which shafts the maximum hand and related resetting elements may be mounted whereby there is no need for an undesirable hole in the crystal.
A further feature of the invention lies in the means for resetting the maximum hand to its starting point. This means includes a pair of magnets one of which is disposed on an inner one of the stub shafts adjacent the maximum hand, and another of which is in the form of a manually operable knob mounted upon an outer one of the stub shafts. The magnets are cooperable with one another in effecting resetting of the maximum hand.
Another feature lies in the provision of a friction washer between the inner magnet and the crystal, whereby friction developed by the washer between the inner magnet and the crystal prevents the maximum hand from deviating from its advanced position under the stress of the usual vibrations to which the instrument is subjected.